The Advanced Television Systems Committee (ATSC) published a Digital Television Standard in 1995 as Document A/53, hereinafter referred to simply as “A/53” for sake of brevity. Annex D of A/53 titled “RF/Transmission Systems Characteristics” is particularly incorporated by reference into this specification. A/53 prescribes a vestigial-sideband (VSB) amplitude-modulation (AM) transmitter modulated by an 8-level digital signal superposed on a residual pilot carrier. The least significant bit (LSB) of 2-bit symbols from convolutionally byte-interleaved Reed-Solomon-coded data packets are subjected to one-half-rate convolutional coding to generate 2/3 trellis coding composed of 3-bit symbols descriptive of respective samples of the 8-level digital signal. This sort of DTV transmitter is commonly referred to as an “8-VSB” DTV transmitter.
In the beginning years of the twenty-first century, efforts were made to provide for more robust transmission of data over broadcast DTV channels without unduly disrupting the operation of so-called “legacy” DTV receivers already in the field. These efforts culminated in an ATSC standard directed to broadcasting data in serial concatenated convolutional coding (SCCC) format to mobile receivers being adopted on 15 Oct. 2009. This standard, referred to as “A/153”, is incorporated by reference within this specification. The data for concatenated convolutional coding are commonly referred to as “MH data” in reference to the mobile and handheld receivers that will receive such data. The M/H data are encapsulated in special format MPEG-2 data packets, referred to as “MHE packets”, MPEG being an abbreviation for “Motion Pictures Experts Group”. The SCCC employs the one-half-rate convolutional coding used to generate 2/3 trellis coding as inner convolutional coding, together with one-half-rate outer convolutional coding, 2-bit symbols of which are block-interleaved before the one-half-rate inner convolutional coding.
In DTV broadcasting as specified in A/153 and later versions of A/53 the most significant bits (MSBs) of the three-bit symbols of 2/3 trellis coding are each pre-coded before mapping the three-bit symbols into 8-level 8-VSB symbols. These MSBs are referred to in A/53 and A/153 as Z-sub-2 bits, the bits of intermediate significance being referred to as Z-sub-1 bits, and the least significant bits (LSBs) being referred to as Z-sub-0 bits. The MSBs are applied as a first of two input signals to an exclusive-OR gate, the response of which besides being the pre-coder response is delayed twelve symbol epochs and applied as a second of the two input signals to the exclusive-OR gate. The pre-coding and the half-code-rate trellis coding of the less significant bits of the three-bit symbols complement a comb-filtering procedure that is performed in a legacy DTV receiver. In this comb-filtering procedure, designed to suppress interference from co-channel NTSC signals, the 8-VSB symbols were supplied as minuend input signals to an analog-regime subtractor and after being delayed twelve 8-VSB symbol epochs were supplied as subtrahend input signals to that subtractor. Principal energy components of an NTSC co-channel interfering signal would be suppressed in the difference output signal, which is data-sliced with a 15-level data slicer. The data slicing results are then converted to a modulo-8 arithmetic to reproduce the three-bit symbols of the 2/3 trellis coding. I.e., in effect, the analog-regime subtractor and the 15-level data slicer are employed as a modulo-8 digital-regime subtractor.
Using an analog-regime comb filter to suppress co-channel NTSC signal is known to degrade the noise performance of the receiver. Although the spacing between data-slicing levels is unchanged by the comb filter, the noise variances of the minuend and subtrahend signals combine vectorially in the difference signal to generate noise variance ranging 3 dB larger in amplitude peaks than in the input signal to the comb filter. However, statistically, the rate of maximum peaks is lower in the difference signal than in the minuend and subtrahend signals, so SNR reduction is on average less than 3 dB. If these peak variance conditions occur infrequently, the trellis decoding procedures will diminish their effect upon decoding results. Nonetheless, pre-coding Z-sub-2 bits will cause some direct degradation of noise performance in M/H receivers and might be better avoided. The same is true for a comb filter realized in a digital regime that over-samples the 8-level symbols two-to-one or more. The degradation of the noise performance of the receiver obtains even in the absence of an NTSC co-channel interfering signal.
Modulo-8 subtraction in the digital regime also clouds issues as to which bits of 2/3 trellis coding are most likely to be in error according to the results of data-slicing the plural-level 8-VSB symbols. At least two of the 3-bit symbols of the 2/3 trellis coding are affected by a single 8-VSB symbol exhibiting a large variance from the norm during data slicing. Trellis decoding helps to resolve such issues, but they might be better avoided by not pre-coding Z-sub-2 bits. This effect destroys the spectral flatness of the additive Gaussian noise, “coloring” it. This complicates trellis coding, which is designed for optimally decoding digital symbols accompanied by additive white Gaussian noise (AWGN).
The intrusion of the modulo-8 subtraction between data slicing and 2/3 trellis decoding vitiates one of the principal strengths of 2/3 trellis coding of eight-level symbols—namely, that the decoding procedure is relevant not just to the two bits directly involved in the half-rate trellis coding, but further extends to the other bit not directly involved. Resolution of the value of a Z-sub-1 bit by 2/3 trellis decoding has implications with regard to the resolution of the Z-sub-2 bit, if those two bits are paired within the mapping of the 2/3 trellis code symbols to the eight-level symbols. The intrusion of the modulo-8 subtraction between data slicing and 2/3 trellis decoding interferes with the pairing of the Z-sub-2 and Z-sub-1 bits within the mapping of the 2/3 trellis code symbols to the eight-level symbols. Gray-code labeling of the outer convolutional coding relies on pairing of the Z-sub-2 and Z-sub-1 bits within the mapping of the 2/3 trellis code symbols to 8-level symbols for 8-VSB. A/153 does not prescribe Gray-code labeling of the outer convolutional coding, but this procedure can halve adjacent-bin errors of the Z-sub-1 bits during data slicing procedures. Gray-code labeling of the outer convolutional coding makes the Z-sub-1 bits substantially as robust as the Z-sub-2 bits Insofar as the decoding of the outer convolutional coding is concerned.
Pre-coding of Z-sub-2 bits in the M/H signals impairs the usefulness of short sequences of 8-VSB symbols encoding M/H data in CCC. The 2/3 trellis coding used as inner convolutional coding is continuous in nature across the successively transmitted segments of fields of interleaved 8-VSB symbols. There are no breaks in this inner convolutional coding caused by the intrusion of 8-VSB symbols encoding main-service data which A/153 specifies similarly to A/53. In some segments of the fields of interleaved 8-VSB symbols, the symbol-interleaved outer convolutional coding is not interrupted by the intrusion of one or more 8-VSB symbols encoding ordinary data. However, in others segments of the fields of interleaved 8-VSB symbols, the symbol-interleaved outer convolutional coding is fragmented by intrusions of 8-VSB symbols encoding ordinary data. It is desirable in the decoding of the symbol-interleaved outer convolutional coding that its fragments be consolidated into a continuous stream of symbols uninterrupted by intrusions of 8-VSB symbols encoding ordinary data, with each successive fragment of the symbol-interleaved outer convolutional coding seamlessly joined to the previous one. Such seamless joinder is imperfectly accomplished if the Z-sub-2 bits in the 8-VSB symbols encoding M/H data are pre-coded, so as not to be independent of the Z-sub-2 bits in the 8-VSB symbols encoding main-service data.
Pre-coding of Z-sub-2 bits in the M/H signals constrains the outer convolutional coding of the M/H data so as to confine the CCC to being serial concatenated convolutional coding (SCCC). SCCC has been preferred by some DTV system designers over parallel concatenated convolutional coding (PCCC) because it is less apt to exhibit a phenomenon called “bit-error-rate floor” or “BER floor” in which bit-error rate (BER) is slow to be reduced in later iterations of turbo decoding procedure. However, PCCC signals can be successfully decoded at lower SNR than SCCC signals can. U.S. Pat. No. 7,310,768 granted 18 Dec. 2007 to D. B. Eidson, A. Krieger and R. Murali of Conexant Systems, Inc. is titled “Iterative decoder employing multiple external code error checks to lower the error floor”. The abstract suggests that cyclic-redundancy-check (CRC) or Reed-Solomon (RS) codes can be used to improve the performance of turbo decoding procedures with regard to overcoming the BER floor phenomenon. The CRC or RS codes can be used to check whether or not strings of data bits in the results of decoding outer convolutional coding are presumably correct. Those strings of data bits indicated very likely to be correct can have the confidence levels associated with their parent soft bits heightened. Re-interleaving will scatter the parent soft bits descriptive of data that have the heightened confidence levels throughout the extrinsic information fed back via the turbo loop, to be used in the next iteration of decoding of inner convolutional coding. This general approach to solving “bit-error-rate floor” problems reduces objection to using PCCC, rather than SCCC. The outer convolutional coding and the inner convolutional coding in PCCC are independent of each other, except for coding the same data. This enables PCCC to reduce BER in fewer iterations than SCCC can, as well as permitting successful decoding at a few tenths dB lower SNR than is possible with SCCC. The independence of outer convolutional coding from inner convolutional coding in PCCC facilitates the decoding of that outer convolutional coding being wrapping around in each M/H Group as described in U.S. patent application Ser. No. 12/924,074 filed by A. L. R. Limberg on 20 Sep. 2010 and titled “Terminated concatenated convolutional coding of M/H Group data in 8VSB digital television signals”.
With the 2008 demise of high-power NTSC broadcasting in the United States and the subsequent curtailment of high-power NTSC broadcasting in Canada and in Mexico, there is little if any need for comb filtering to suppress interference from co-channel NTSC signals. Even so, A/153 prescribed continued use of the pre-coding of the MSBs of the three-bit symbols of 2/3 trellis coding that are mapped into 8-level 8-VSB symbols. The proffered rationale for this was that many legacy receivers were not equipped for decoding 8-VSB in which the Z-sub-2 bits were not pre-coded. Legacy DTV receivers are not equipped for decoding M/H signals, whether or not the Z-sub-2 bits in the M/H signals are pre-coded. So long as the ordinary 8-VSB signals authorized by A/53 as originally published in 1995 use pre-coding of Z-sub-2 bits, legacy DTV receivers will continue to receive ordinary 8-VSB signals as originally specified by A/53. This suggests that selectively discontinuing pre-coding of Z-sub-2 bits just for M/H signals should have no deleterious effects for receivers designed just to receive ordinary 8-VSB signals as originally specified by A/53.
However, simply selectively discontinuing pre-coding of Z-sub-2 bits just for M/H signals can discommode legacy DTV receivers that estimate the signal-to-noise ratio (SNR) of received DTV signals by counting the number of (207, 187) Reed-Solomon codewords per data field or frame that are correct or correctable. Post-comb filtering in these legacy receivers mutilates the (207, 187) RS codewords for MHE packets, so that the RS decoder in such a legacy DTV receiver is likely to find all or almost all of them to be in error. The number of RS codewords per data field or frame that will found to be in error becomes large enough to cause such a legacy DTV receiver to conclude that the SNR of the received DTV signal is too low to be useful. Accordingly, the receiver is de-activated.
Provisional U.S. Pat. App. Ser. No. 61/337,680 filed 11 Feb. 2010 by A. L. R. Limberg and titled “Coding and decoding of 8-VSB digital television signals intended for reception by mobile/handheld receivers” describes selective pre-coding procedures that avoid the problem of unwanted de-activation of legacy DTV receivers. The Z-sub-2 bits of bytes from RS-coded main-service packets are pre-coded, together with the Z-sub-2 bits of the initial two bytes from each MHE packet. The convolutionally byte-interleaved RS codewords as so selectively pre-coded are then post-comb filtered and de-interleaved. This recovers the RS codewords in the form in which a DTV legacy receiver would receive them for RS decoding were no further steps taken in the M/H DTV transmitter to avoid erroneously RS-coded MHE packets appearing in the de-interleaved post-comb filter response. The RS-coded main-service packets are recovered as valid (207, 187) RS codewords free of any error. However, the RS-coded M/H-service packets that are recovered are very unlikely to be valid (207, 187) RS codewords, owing to their having been post comb-filtered without previous pre-coding of the Z-sub-2 bits in most of their bytes. The apparent error in the RS-coded M/H-service packets is ascribed to inappropriate RS parity bytes, and the transmitter replaces these inappropriate RS parity bytes by recalculated RS parity bytes. The bytes of the M/H data as they appear in the de-interleaved post-comb filter response are considered to be free of error and will be restored to their original condition during the subsequent modified 2/3 trellis coding.
Provisional U.S. Pat. App. Ser. No. 61/335,246 filed 4 Jan. 2010 by A. L. R. Limberg and titled “Coding and decoding of RS frames in 8-VSB digital television signals intended for reception by mobile/handheld receivers” describes other selective pre-coding procedures that avoid the problem of unwanted de-activation of legacy DTV receivers. These other selective pre-coding procedures differ from those described in U.S. Pat. App. Ser. No. 61/337,680 in that no RS coding step precedes the step of selective pre-coding of Z-sub-2 bits and the subsequent step of post-comb filtering. RS coding steps are deferred until after the steps of selective pre-coding and post-comb filtering. The alternative procedures for selectively pre-coding Z-sub-2 bits that are described in U.S. Pat. App. Ser. No. 61/335,246 and in U.S. Pat. App. Ser. No. 61/337,680, respectively, are based on the same insight. Namely, the RS coding of the MHE packets is based on the form that the bytes of those packets appear in after post-comb filtering in a legacy DTV receiver.
Simply discontinuing pre-coding of Z-sub-2 bits for M/H signals presents another problem for DTV receivers, as noted by C. H. Strolle et alii in A1 U.S. publication No. 2004-0028076 of 12 Feb. 2004 titled “Robust data extension for 8-VSB signaling”. The problem is that of the receiver having to restore the correct sense of logic for main-service signal each time it resumes after the intrusion of M/H-service signal. The selective precoding procedures described herein and previously disclosed in U.S. patent application Ser. Nos. 61/335,246 and 61/337,680 provide for continuous pre-coding of the Z-sub-2 bits of the multiplexed main-service and M/H-service components of the transmitted 8-VSB signal. The DTV receiver does not have to pursue particular measures for maintaining the correct senses of logic for the Z-sub-2 bits of the main-service data and the M/H-service data. The correct senses are maintained automatically.